Sensitized photoconductive compositions comprising zinc oxide



March 17, 1964 v s w 3,125,447:-

SENSITIZED PHOTOCONDUCTIVE COMPOSITIONS COMPRISING ZINC OXIISE Filed NOV. 25, 1960 Fig. 1

llll III! M Wm 1m III 2-1 1 -D|METHYLAM|NOSTYRYL) 3 s- CARBOXYETHYL- THIAZOLIUM IODI DE 2-(1-7-DIMETHYLAMINOSTYRYQ-I' B'CARBOXYETHYL" PYRIDINIUM IODI DE Fig3 TOO 600 500 400 m) I 2-[4-(2-cARBoxYAmLmo) -|,a- BUTADIENYL] 3-ETHYLBENZOTH|AZOL|UM IODIDE Paul III-Stewart ATTORNEYS United States Patent 3,125,447 SENSITIZED PHOTOCONDUCTIVE COMPOSETION COMPRISING ZINC OXIDE Paul H. Stewart, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Nov. 25, 1960, Ser. No. 71,569 11 Claims. (Cl. 96-1) This invention relates to spectrally sensitized photoconductive layers comprising zinc oxide which are particularly usefulin making photographic copies (black and white, or color), and a method of making such photoconductive compositions.

This application is a continuationsin-part of my application Serial No. 630,461, tiled December 26, 1956, now forfeited.

It is known that zinc oxide can be employed in making photoconductive layers on ordinary paper and that photographic copies can conveniently be prepared from these photoconductive papers. This process has been described as being somewhat similar to the system known as xerography in that a photoconductive plate is employed and after exposure of the plate to a photographic image, development of the latent image is accomplished by means of a pigmented resinous composition which adheres to the unexposed portions of the exposed plate. However, the xerographic plate is generally used to transfer the developed image to a receiving sheet, whereas the known system for using photoconductive zinc oxide generally makes use of the zinc oxide 'layer itself as a means of providing the desired photographic copy without transfer of any electrostatic charge to a receiving sheet.

In the known system of employing zinc oxide in photoconductive layers, the grounded support, which is generally paper, is first made sensitive to light by giving it a blanket negative electrostatic charge on the zinc oxide layer in the substantial absence of any ultraviolet or visible radiation. "One convenient means of giving the zinc oxide layer an electrostatic negative charge is by means of ion transfer from a corona discharge. The zinc oxide photoconductive layer can then be exposed to a photographic image in the usual manner, the portions of the zinc oxide which receive light or ultraviolet radiation losing wholly, or in part (depending upon extent of exposure), the negative electrostatic charge, while the unexposed portions of the photoconductive layer retain their negative electrostatic charge. The resulting latent image can then be developed by means of a pigmented resin powder which has a charge opposite to the negative charge a of the unexposed areas of the photoconduetive layer. The pigmented powder is thus firmly attached or attracted to the negatively charged areas. The pigmented resin powder can then be affixed to the photoconductive layer by simply melting the resinous vehicle at a temperature below the charting temperature of the paper support, so that the resinous powder becomes fused to the surface of the original photoconductive layer. Various means of developing the latent image in the .photoconductive layer to a visible image have been described in the prior art.

One disadvantage in the zinc oxide normally used in such photoconductive layers is that the light-sensitivity of such charged zinc oxide normally is at its greatest in the ultraviolet region of the spectrum, whereas the exposing source may have its maximum output in a region of the spectrum which lies within the visible region, such as an ordinary tungsten light. While various means have been previously described for sensitizing the zinc oxide so that it has some panchromatic or orthochromatic sensitivity, for example, by means of various organic dyes such as Rose Bengal, and the like, these methods have not been particularly satisfactory since the disadvantage of strongly dyeing the zinc oxide layer more than offsets the sensitivity which is supplied to the teebly sensitive zinc oxide. That is, it is generally desirable to have some means of sensitizing the zinc oxide which does not permanently color the zinc oxide layer, which might be used as the final copy of the photographic image. Strong coloration of the zinc oxide layer has an unfavorable aesthetic effect and might be strongly objectionable in the even-t that it is desired to make color prints of the original subject. Other unfavorable effects, such as poor contrast, slow speeds, etc., are evident.

An object of my invention is to provide a convenient means of spectrally sensitizing photoconductive zinc oxide layers in a useful manner. Another object is to provide a means of spe-ct-rally sensitizing photoconductive zinc oxide layers by means of styryl dyes or hemicyanine dyes. Still another object is to provide particular classes of such dyes which are out-standing in their spectral sensitizing properties tor photoconductive zinc oxide layers. Other objects will become apparent from a consideration of the following description and examples.

My invention is illustrated graphically in the accompanying drawing where FIGURES 1 to 3 are diagrammatic reproductions of spectrograms of photoconductive zinc oxide compositions sensitized according to my invention.

The spectral sensitizing dyes which I have found to be particularly useful in sensitizing photoconductive zinc oxide layers comprise dyes known as hemicyanine dyes and styryl (phenylvinyl) dyes. The spectral sensitizing dyes of my invention include the dyes represented by the following general formula:

n-rfizo H--CH) t: (L=L) d :a

wherein R represents an alkyl group (e.g., methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, n-amyl, isoamyl, etc.), a hydroxyalkyl group (e.g., fi-hydroxyethyl, fi-hydroxypropyl, y-hydroxypropyl, etc.), an alkoxyalkyl group (e.g., ,B-methoxyethyl, fl-ethoxyethyl, etc.), a carboxyalkyl group (e.g., carboxymethyl, ,B-carboxyethyl, m-carboxyethyl, 'y-carboxypropyl, e-carboxybutyl, etc.), a sulfoalkyl group (e.g., sulfomethyl, fl-sulfoethyl, 'y-sulfopropyl, fi-sulfobutyl, etc.), a carbalkoxyalkyl group (e.g., carbomethoxymethyl, ,B-carbomethoxyethyl, carbethoxymethyl, B-carbethoxyethyl, etc.), an acyloxyalkyl group (e.g., B-acetoxyethyl, 'y-acetoxypropyl, etc.), an aralkyl group (e.g., benzyl, ,B-phenethyl, etc.), etc., R represents an amino group (e.g., amino, monomethylamino, dimethylamino, monoethylamino, diethylamino, dibutylamino, anilino, 0-, mand p-carhoxyanilino, morpholino, piperidyl, etc.), or an aminoaryl group (e.g., aminophenyl, dimethylaminophenyl, diethylamin'ophenyl, -dibutylaminophenyl, aminonaphthyl, etc.), L represents a methine group, i.e., -CR'=, wherein R represents a hydrogen atom, an alkyl group (e.g., methyl, ethyl, etc.), an alkoxyl group (e.g., methoxyl, ethoxyl, etc.), or an aryl group (e.g., phenyl, tolyl, etc.), X represents an acid radical, such as chloride, bromide, iodide, perchlorate, acetate, thiocyanate, benzenesulfonate, p-toluenesulfonate, methylsulfate, ethylsulfate, etc., n and d each represent a positive integer of from 1 to 2, and Z represents the nonmetallic atoms necessary to complete a heterocyclic nucleus containing from 5 (n is 1) to 6 (n is 1 or 2) atoms in the heterocyclic ring of the type customarily employed in the cyanine dye art, provided the above formulated dyes contain at least one carboxyl radical.

Typical heterocyclic nuclei as defined by Z above inelude a thiazole nucleus (e.g., thiazole, 4-metl1ylthiazole, S-methylthiazole, 4-phenylthiazole, S-phenylthiazole, 4,5- dimethylthiazole, 4,5-diphenylthiazole, 4-(2-thienyl)thiazole, etc.), a benzothiazole nucleus (e.g., benzothiazole, 4-chlorobenzotl iazole, S-chilorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, 6-methylbenzothiazole, 5- bromobenzothiazole, 6 bromobenzothiazole, 4 phenylbenzothiazole, 5-phenylbenzothiazole, 4 methoxybenzotbiazole, S-methoxybenzothiazole, 6-methoxybenzothiazole, 5-iodobenzothiazole, 6-iodobenzothiazole, 4-ethoxybenzothiazole, 5 -ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6-dimethoxybenzothiazole, 5,6 -dioxymethylenebenzothiazole, 5 hydroxybenzothiazole, 6 hydroxybenzothiazole, 5-carboxybenzothiazole, etc), a naphthothiazole nucleus (e.g., a-naphthothiazole, fi-naphthothiazole, S-methoxy-fi-naphthothiazole, S-ethoxy-[i-naphthothiazole, 7 methoxy-ac-naphthothiazole, 8 I11thOXY-Otnaphthothiazole, etc), a thianaphtheno-7',6,4,5-thiazole nucleus (e.g., 4-meth0xythianaphtheno-7',6,4,5-thiazole, etc.), an oxazole nucleus (e.g., 4-methyloxazole, S-inethyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, 5-phenyloxazole, etc), a benzoxazole nucleus (e.g., benzoxazole, S-chlorobenzoxa- Zole, S-phenylbenzoxazole, S-methylbenzoxazole, 6-methylbenzoxazole, 5,6- dimethylbenzoxazole, 4,6 dimethylbenzoxazole, 5-methoxybenzoxazole, 6-methoxybenzoxazole, S-ethoxybenzoxazole, 6 chlorobenzoxazole, 5 hydroxybenzoxazole, 6-hydroxybenzoxazole, S-carboxybenzoxazole, etc.), a naphthoxazole nucleus (e.g., a-naphthoxazole, ,B-naphthoxazole, etc.), a selenazole nucleus (e.g., 4-metl1ylselenazole, 4-phenylselenazolej, etc), a benzoselenazole nucleus (e.g., benzoselenazole, S-chlorobenzoselenazole, 5-methoxybenzoselenazole, S-hydroxybenzoselenazole, tetrahydrobenzoselenazole, etc), a naphthoselenazole nucleus (e.g., u-naphthoselenazole, B-naphthoselenazole, etc.), a thiazoline nucleus (e.g., thiazoline, 4-methylthiazoline, etc.), a 2-quinoline nucleus (e.g., quinoline, S-methylquinoline, S-methylquinoline, 7-methylquinoline, 8 methylquinoline, 6 chloroquinoline, 8- chloroquinoline, 6-methoxyquinoline, fi-ethoxyquinoline, 6-hydroxyquinoline, 8-hydroxyquinoline, etc.), a 4-quinoline nucleus (e.g., quinoline, 6-methoxyquinoline, 7- methylquinoline, 8-methylquinoline, etc.), a l-isoquinoline nucleus (e.g., isoquinoline, 3,4-dihydroisoquinoline, etc.), a 3,3-dialkylindolenine nuleus (e.g., 3,3-dimethylindolenine, 3,3,5 trimethylindolenine, 3,3,7 trirnethylinodolenine, etc.), a Z-pyridine nucleus (e.g., pyridine, 3- methylpyridine, 4-methylpyridine, S-rnethylpyridine, 6- methylpyridine, 3,4-dimethylpyn'dine, 3,5 dimethylpyridine, 3,6-dimethylpyridine, 4,5-dimethylpyridine, 4,6-dimethylpyridine, 4- chloropyridine, 5 chloropyridine, 6- chloropyridine, 3-hydroxypyridine, 4-hydroxypyridine, 5- hydroxypyridine, 6-hydroxypyridine, B-phenylpyridine, 4- phenylpyridine, 6-phenylpyridine, etc), a 4-pyridine nucleus (e.g., Z-methylpyridine, 3-methylpyridine, 2- chloropyridine, 3 chloropyridine, 2,3 dimethylpyridine, 2,5dimethylpyridine, 2,6 dimethylpyridine, 2 hydroxypyrridine, B-hydroxypyridine, etc.), etc.

The carboxyl-substituted dyes of my invention are markedly superior to the corresponding carboxyl-free dyes, as hereinafter shown. Dyes containing a carboxyethyl group have been found to be particularly useful.

Among the more useful styryl dyes useful in practicing my invention are those represented by the following general formula:

Q R-hl (:CH -CH) Fl-C (CH=CH) 1- -N\ represents a carboxyalkyl group, although the carboxy radical can be present at some other location in the molecule, for example, on a benzene ring in the dyes wherein Z represents the residue of an arylene heterocyclic nucleus.

Particularly useful hemicyanine dyes useful in practicing my invention include those represented by the following general formula:

(III) R R4 wherein R, X, n, a and Z each have the values given above, R represents a hydrogen atom, an alkyl group (e.g., methyl, ethyl, n-propyl, n-butyl, etc., especially an alkyl group containing from 1 to 4 carbon atoms) or an aryl group (e.g., phenyl, tolyl, chlorophenyl, methoxyphenyl, ethoxyphenyl, carboxyphenyl, aminophenyl, dimethylarninophenyl, diethylaminophenyl, naphthyl, etc., especially a monocyclic aryl group of the benzene series), R represents an alkyl group, including those alkyl groups enumerated above with respect to R or an aryl group, including those listed above with respect to R or, alternatively, R and R together represent the non-metallic atoms necessary to complete a heterocyclic nucleus containing from 5 to 6 atoms in the heterocyclic ring (e.g., piperidyl, morpholinyl, pyrrolinyl, indolininyl, etc.), provided such dyes contain at least one free carboxyl group. Advantageously, this free carboxyl group or radical can be attached to the R moiety or to a carbon atom of the R or R moiety.

Sometimes the dyes of Formula II or III spontaneously lose the element of HX in those cases where R represents a carboxyalkyl group. Such dyes are known as anhydronium bases or hydroxides, and it is to be understood that whereas the dyes of Formulas II and III have been depicted as containing a conventional acid anion, my invention contemplates such dyes which are present in the form of their anhydronium bases or hydroxides and, consequently, do not contain the X moiety.

Hemicyanine dyes and styryl dyes useful in practicing my invention include, for example, the following:

2-[ l-(p-carboxyanilino)-l,3-butadienyl1-3-ethylbenzoselenazolium iodide 2-[4-(o-carboxyanilino)-1,3-butadienyl]-3-ethylbenzothiazolium iodide Z-(p-diethylarninostyryl)-3-carboxymethylbenzothiazolium bromide 2-(2-analinovinyl)-3-carboxymethylbenzoxazolium iodide 2- [2-( l-pipcridyl) vinyl] 1-;3-carb0xyethyl-flnaphthothiazolium bromide 2-(p-dimethylaminostyryl)-3-carboxymethyl-benzoxazolium iodide Anhydro-Z- p-dimethylaminostyryl) -3 -carboxymethylbenzothiazolium hydroxide 2-(p-dimethylaminostyryl)-1-carboxymethylquinolinium iodide 2-(p-dimethylaminostyryl) -3-,B-carboxyethylthiazolinium iodide 2- (p-dimethylarninostyryl l-B-carboxyethylpyridinium iodide 4- p-dimethylaminostyryl 1 -fi-carboxyethylquinolinium iodide 2-(p-dimethylaminostyryl)-6-nitro-3-fl-carboxyethylbenzothiazolium iodide 2- [p-diethylaminophenyl) 1 ,3-butadienyl] -3-carboxymethyloenzothiazolium bromide 2-(Z-N-rnethylanilino)-vinyl-3-fi-carboxyethylbenzoxazolinium bromide S-carb oxy-2- (p-diethylaminostyryl) -3 -ethylbenzox azolinium iodide S-carboxy-Z- [4-(anilino -l ,3-butadienyl -3-ethylbenzothiazolium bromide Methods for making hemicyanine dyes useful in practicing my invention have been previously described in the cyanine dye art. Among the patents describing general methods for making such dyes are the following:

US. 2,166,736, issued on July 18, 1939 US. 2,263,749, issued onNov. 25, 1941 US. 2,301,361, issued on Nov. 10, 1942 U.S.2,369,509, issued on Feb. 13, 1945 US. 2,500,127, issued on Mar. 7, 1950 U.S. 2,500,128, issued on Mar. 7, 1950 Methods for making styryl dyes'useful-in' practicing my invention have also been previously described in the prior art, and among patents describing 'such'methods are'the following:

U.S. 1,845,404, issued on Feb. 16, 1932 U.S. 1,942,854, issued on Jan. 9, 1934 US. 2,231,658, issued on Feb. 11, 1941 US. 2,494,032, issued on Jan. 10, 1950 EXAMPLE A 2-(4-p-Carboxyanilino-Z,S-Butadienyl) -3Ethylbenz0- selenazolium iodide A mixture of 13.08 g. (1 mol.) of 2-(4-acetanilido-L3- butadienyl)-3-ethylbenzoselenazoliurn iodide and 6.85 g. (1 mol., +100%) of p-aminobenzoic acid were heated together in 100 ml. of anhydrous ethanol at the refluxing temperature for 30 minutes. After cooling to room temperature, the solid was collected on a filter and washed with methanol. The residue was transferred to a beaker, stirred in boiling methanol and the suspension was filtered hot. The yield of dye was 62% after two recrystalliza tions from methanol. The blue needles melted at 256- 257 C. with decomposition.

Useful styryl dyes can be prepared by condensing an arene aldehyde containing an amino group, such as dimethylaminobenzaldehyde or dimethylaminocinnamaldehyde with a cyclammonium quaternary salt containing a methyl group in a reactive position. (See, for example, US. Patent 2,231,658.)

The abovespectral sensitizing dyes can be combined with the photoconductive zinc oxide material in any convenient manner. For example, the spectral sensitizing dye can be added to the Zinc oxide composition while dissolved in an organic solvent. Pyridine, methanol, ethanol, acetone, etc., can be used to dissolve many of the dyes useful in practicing my invention. The zinc oxide can be uniformly dispersed in an organic solution of the binder customarily employed for the zinc oxide and a solution of the dye added to this coating solution. After thorough mixing, the sensitized solution can be coated on a paper support and dried in the usual manner.

Alternatively, an unsensitized zinc oxide coating can he prepared as described above and after removal of the organic solvent, the paper coating can be immersed in a solution (organic or aqueous) of the sensitizing dye. This method has been found to be particularly useful in that higher speeds can be frequently obtained.

The binders for the zinc oxide comprise many of the resinous compositions which are commercially available.

Such resins are sold under trade names, such as Plaskon ST856, Rezyl 405-18, Pliolite S-7, Styresol 4440, DC 804, etc. These resins comprise styrene-butadiene copolymers, silicone resins, styrene-alkyd resins, siliconealkyd resins, soya-alkyd resins, polyvinyl chloride, polyvinyl acetate, etc. The methods of making such resins have been previously described in the prior art. For example, styrene-alkyd resins can be prepared according to the method described in US. Patent 2,361,019, issued October 24, 1944; U.S. Patent 2,258,423, issued October 7, 1941; U8. Patent 2,453,665, issued November 9, 1948, etc. Other'binders, such as parafiin, mineral waxes, etc., can also be employed. These binders are generally characterized as having marked hydrophobic properties (i.e., being substantially free of any water-solubilizinggroups, such as hydroxyl, free acid groups, amide groups, etc.) and as being good electrical insulators or as having high electrical resistivity. These binders can be easily dis solved in organic solvents having a boiling point below the charring temperature of the paper support. Also, these binders have the desirable property of readily dispersing the zinc oxide photoconductive material. Some resinous binders are relatively poor insulators and do not provide coatings which can be stored for prolonged periods of times after the photoconductive coatings have been negatively charged. This is particularly noticeable at relatively high humidities, and the photoconductive coatings should be negatively charged shortly before use in such instances, that is, it is not advisable to charge the photoconductive coatings too long in advance before use. Such problems are well understood by those skilled in the art.

The zinc oxide photoconductive material employed in my invention should generally consist of relatively small particles of less than 0.5 micron mean diameter. Such Zinc oxide materials are readily available and can be purchased under a variety of trade names, such as Protox No. 168 (New Jersey Zinc Company), etc. Suflicient binder should be employed to insulate each of the zinc oxide particles from the surrounding particles in the composition. The most useful or optimum quantity of zinc oxide to binder for a particular binder can be readily determined by making a series of test coatings wherein the quantity and relative amounts of zinc oxide to binder are employed.

Exposure of the charged photoconductive layer to visible radiation or ultraviolet radiation causes a loss or reduction of the negative charge in those portions of the photoconductive material which are exposed to the radiation. The degree of loss will depend on the intensity and time of exposure to the radiation, in general. The resulting latent electrophotographic image can then be developed to a visible image in a variety of ways, including those which have been previously employed in electrophotographic processes, such as xerography. A particularly useful means of developing the latent electrophotographic image comprises use of a magnetic brush. This magnetic brush development makes use of a ferromagnetic powder, such as iron filings, which has been intimately mixed with pigmented resin, or sulfur. Agitation of the ferromagnetic powder and pigmented resin results in a triboelectric effect wherein the pigmented resin acquires an electric charge depending upon the relative position of the resin to the ferromagnetic powder in the triboelectric series. That is, ordinary iron powder is below most resins in the triboelectric series, and mixture with a resin higher in the series results in the deposition of a positive electrostatic charge on the resin. The resulting mixture can then be picked up by a magnet on which the iron particles, or other ferromagnetic powder, arrange themselves in the conventional pattern, so that the long chains of filings resemble an ordinary brush. This magnetic brush can then be placed in contact with the exposed photoconductive layer and the brush passed across the negative electrostatic latent image which is on the surface of the photoconductive material. As the magnetic brush passes over the areas of the photoconductive material which have residual negative charge thereon, the electrostatic attraction between the charged pigmented resin particles and the oppositely charged image areas in the photoconductive material is greater than the attraction between these particles and the ferromagnetic powder, so that the pigmented resin is deposited on the surface of the photoconductive material roughly in proportion to the residual charge on the surface of the photoconductive layer. By selecting a resin with a low melting point, the developed image can then be fixed to the surface of the paper by heating to a temperature above the melting point of the resin, but below the charring temperature of the paper. The resin in the pigmented resin compositions can be varied, depending upon the effects desired and the type of copy which is being reproduced. Such resins may be the same as those employed in the insulating layer coated on the paper support, such as styrene-butadiene resin, etc. The particle size of the pigmented resin used in development can vary, although the range of 0.1 to microns is adequate for most purposes. Various pigments can be used in the resin developing compositions. The ability of the pigmented resin to accept a positive charge is dependent upon the type of resin selected. The pigment merely serves to impart color to the resin and probably imparts very little, if any, influence on the over-all charge of the pigmented resin.

The following examples will serve to illustrate more fully the manner of preparing and using the spectrally sensitized photoconductive zinc oxide materials of my invention.

A number of the dyes represented by the above Arabic numbers were dissolved in a suitable solvent, such as acetone, water, pyridine, methanol, etc, depending upon the solubility characteristics of the particular dye, and separate sheets of paper coated with unsensitized photoconductive zinc oxide compositions were then immersed in these solutions containing the sensitizing dyes. The sensitized photoconductive zinc oxide papers were then dried in air in a vertical position. After drying, the strips were charged under a corona discharge and exposed for one-half second in a sensitometer using tungsten illumination. The exposed coatings were then developed by the magnetic brush technique described above, using small iron particles and black pigmented sulfur. Finally, the images were fixed by fusing the black pigmented sulfur to the surface by applying heat. A duplicate strip treated in the same manner, but with the sensitizing dye being omitted, served as a control. Table I below summarizes the results obtained with several of the spectral sensitizing dyes identified above.

Table I Percent Concentration of Dye Solution by Weight Dye N0. Relative White Light Speed 4% suitable solvent (ethanol) and photoconductive zinc oxide papers immersed in these solutions, dried and exposed in the manner described above with respect to Table I. The results obtained according to this procedure are summarized in Table II below.

Table 11 Percent Con- Dye N0. ecntration of Relative White Dye Solution Light Speed by Weight Control 01 400 01 1, 000 01 1, 000 01 1, (J00 01 800 01 1, 000 01 1, 000

The dyes of my invention can be used in the form containing free carboxyl groups, or as salts of such groups, e.g., triethylammonium, sodium, pyridinium, triethanolammonium, etc), since it has been found that the dyes are absorbed to the surface of the zinc oxide in the form of their anions.

The accompanying drawing illustrates schematically the increased spectral range of sensitivity provided by three of the dyes useful in practicing my invention. In FIGURES 1, 2 and 3, the solid curves show the range of sensitivity as well as the region of maximum sensitivity. Exposures were made in a spectograph in the usual manner employing tungsten illumination.

In FIGURE 1, the solid curve represents the sensitivity of a photoconductive element comprising a paper support having coated thereon a relatively thin layer of photoconductive zinc oxide sensitized with Z-(p-dimethylaminostyryl)-3 B carboxyethylthiazolium iodide. The relative speed of this coating corresponds to that given above in Table II for dye 9.

In FIGURE 2, the solid curve represents the sensitivity of a photoconductive element comprising a paper support having coated thereon a relatively thin layer of photoconductive zinc oxide sensitized with 2-(p-dimethylaminostyryl) 1 B carboxyethylpyridinum iodide. The relative speed of this coating corresponds to that given above in Table II for dye 10.

In FIGURE 3, the solid curve represents the sensitivity of a photoconductive element comprising a paper support having coated thereon a relatively thin layer of photoconductive zinc oxide sensitized with 2-[4-(o-carboxyanilino)-1,3-butadienyl] 3 ethylbenzothiazolium iodide. The speed found for this coating corresponds to that given in Table I above for dye 2.

It has been found that the dyes of my invention containing a free carboxyl group or radical have a markedly improved sensitizing action toward zinc oxide as compared with the sensitizing action of corresponding dyes containing no such free carboxyl group or radical. This appears to be true regardless of the manner by which the carboxyl group or radical is attached to the molecule of the parent sensitizing dye. In silver halide photography, the carboxyl and carboxyl-free dyes frequently have about the same sensitizing efiiciency. In some instances, it has been found that carboxyl-free dyes have very little sensitizing action for zinc oxide compositions of the type contemplated by the present invention.

The photoconductive zinc oxide employed in my invention can be any of the well known photoconductive zinc oxides employed in electrophotographic or photoconductographic applications. While the above examples have been directed primarily to the sensitization of zinc oxide compositions which are employed in xerographic processes, it is to be understood that the sensitized zinc oxides of my invention can be employed to equal advantage in photoconductography.

Photoconductive zinc oxides useful in the methods of my invention include photoconductive zinc oxides known generally as French process zinc oxides.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be eflected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What I claim and desire secured by Letters Patent of the United States of America is:

1. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric binder-insulator for said photoconductive zinc oxide, and adsorbed to the surface of said photoconductive zinc oxide a spectral sensitizing dye selected from those represented by the following general formula:

wherein R represents a member selected from the class consisting of an alkyl group, a hydroxyalkyl group, an alkoxyalkyl group, a carboxyalkyl group, a sulfoalkyl group, a carbalkoxyalkyl group, an acyloxyalkyl group and an aralkyl group, R represents a member selected from the class consisting of an amino group and an aminoaryl group, L represents a methine group, X represents an acid anion, n and d each represent a positive integer of from 1 to 2, and Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing from 5 to 6 atoms in the heterocyclic ring, provided said sensitizing dye contains at least one carboxyl radical attached to a carbon atom thereof.

2. A photoconductive composition comprising photoconductive zinc oxide, an organic, high dielectric, binderinsulating material for said photoconductive zinc oxide and in contiguity with said photoconductive zinc oxide, 2-(p-diethylaminostyryl) 3 carboxymethylbenzothiazolium bromide.

3. A photoconductive composition comprising photoconductive zinc oxide, an organic, high dielectric, binderinsulating material for said photoconductive zinc oxide and in contiguity with said photoconductive zinc oxide, 2 (p-dimethylaminostyryl) 1 carboxymethylquinolinium iodide.

4. A photoconductive composition comprising photoconductive zinc oxide, an organic, high dielectric, binderinsulating material for said photoconductive zinc oxide and in contiguity with said photoconductive zinc oxide, 2 (p dimethylaminostyryl) 3 ,8 carboxyethylthiazolinium iodide.

5. A photoconductive composition comprising photoconductive zinc oxide, an organic, high dielectric, binderinsulating material for said photoconductive zinc oxide and in contiguity with said photoconductive zince oxide, 2-[4-(p-carboxyanilino) 1,3 butadienyl] -3-ethylbenzoselenazolium iodide.

6. A photoconductive composition comprising photoconductive zinc oxide, an organic, high dielectric, binderinsulating material for said photoconductive zinc oxide and in contiguity with said photoconductive zinc oxide, 2-[4-(o-carboxyanilino) 1,3 butadienyl]-3-ethylbenzo thiazolium iodide.

7. A photographic element suitable for use in electrophotography comprising a conductive support having 10 coated thereon a photoconductive insulating layer comprising photoconductive zinc oxide, a high dielectric, organic, binder-insulating material for said photconductive zinc oxide, and in contiguity with said photoconductive zinc oxide a spectral sensitizing dye selected from those represented by the following general formula:

--"'0Z R-Ni=cH -on ...,-''o =L).1-P.l

wherein R represents a member selected from the class consisting of an alkyl group, a hydroxyalkyl group, an alkoxyalkyl group, a carboxyalkyl group, a sulfoalkyl group, a carbalkoxyalkyl group, an acyloxyalkyl group and an aralkyl group, R represents a member selected from the class consisting of an amino group and an aminoaryl group, L represents a methine group, X represents an acid anion, n and d each represent a positive integer of from 1 to 2, and Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing from 5 to 6 atoms in the heterocyclic ring, provided said sensitizing dye contains at least one carboxyl radical attached to a carbon atom thereof.

8. A photoconductive composition of claim 1 in which the Z group of the spectral sensitizing dye represents the non-metallic atoms required to complete a benzothiazole nucleus.

9. A photoconductive composition of claim 1 in which the Z group of the spectral sensitizing dye represents the non-metallic atoms required to complete a Z-quinoline nucleus.

10. A photoconductive composition of claim 1 in which the Z group of the spectral sensitizing dye represents the non-metallic atoms required to complete a thiazoline nucleus.

11. A photoconductive composition of claim 1 in which the Z group of the spectral sensitizing dye represents the non-metallic atoms required to complete a benzoselenazole nucleus.

References Cited in the file of this patent UNITED STATES PATENTS 1,730,505 Hart Oct. 8, 1929 1,942,854 Brooker Jan. 9, 1934 2,153,928 Kendall Apr. 11, 1939 2,166,736 White et al. July 18, 1939 2,191,810 Stevens Feb. 27, 1940 2,231,658 Brooker et a1. Feb. 11, 1941 2,263,749 White et al. Nov. 25, 1941 2,301,361 Brooker et al. Nov. 10, 1942 2,369,509 White Feb. 13, 1945 2,466,523 White et a1. Apr. 5, 1949 FOREIGN PATENTS 201,416 Australia Apr. 13, 1956 OTHER REFERENCES CA. 43 (1949), 7349d. (Copy in Sci. Lib.)

Young et al.: RCA Review, December 1954, pp. 469- 484. (In. Sci. Lib.)

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1. A PHOTOCONDUCTIVE COMPOSITION COMPRISING PHOTOCONDUCTIVE ZINC OXIDE, A HIGH DIELECTRIC BINDER-INSULATOR FOR SAID PHOTOCONDUCTIVE ZINC OXIDE, AND ADORBED TO THE SURFACE OF SAID PHOTOCONDUCTIVE ZINC OXIDE A SPECTRAL SENSITIZING DYE SELECTED FROM THOSE REPRESENTED BY THE FOLLOWING GENERAL FORMULA: 